Vaccines:
Vaccination is the administration of an antigenic material (the vaccine) to produce immunity to a disease. Vaccines can be used to prevent (i.e. prophylactic use) or ameliorate (i.e. therapeutic use) the effects of a pathology, for example an infection by a pathogen. Vaccination is considered to be the most effective and cost-effective method of preventing infectious diseases. The material administered as an immunogen are generally live but weakened or attenuated forms of pathogens (bacteria or viruses), killed or inactivated forms of these pathogens, or purified material such as proteins.
The antigenic material (also called an immunogen) stimulates a response in an animal against the respective pathogen following the initial administration and also in future encounters with the pathogen, thus providing protection against infection by the pathogen in future exposures. Smallpox was the first disease people tried to prevent by purposely inoculating themselves with other types of infectious agents. For example, the cowpox vaccine was used as an immunization for smallpox in humans by the British physician Edward Jenner in 1769.
However, not all vaccines and the immunogens in them are equally effective in stimulating an effective immune response. For example, poor immunogenicity of vaccines available against tuberculosis (TB), Streptococcal pneumonia (SP), measles virus Edmoston-Zagreb strain (EZMV), meningococci, hemagglutinin (HA) of influenza viruses, and hepatitis B virus have been reported. Moreover, some vaccines require an extended period of vaccination regime before immunity is successfully induced. For example, the primary basic vaccination against Bacillus anthracis requires six doses, three subcutaneous injections in the deltoid at zero, two, and four weeks, and three vaccinations at six, twelve, and eighteen months followed by annual boosters. For prolonged protection, annual boosters are required.
One reason accounting for the poor immunogenicity of some vaccines is the inability of some immunogens to enter the cytosol and the major histocompatibility complex pathway in order to stimulate a cell-mediated immune response. A number of bacterial toxins contain domains that share the ability to gain access to the host cell cytosol, where they can exert their effects. Although each toxin can differ in the mechanism or route by which it gains entry to the cytosol, the overall effect is that of a “molecular syringe” that is able to inject the toxic protein into the cell. Several bacterial toxins, including diphtheria toxin (DT), Pseudomonas exotoxin A (PE), pertussis toxin, and the pertussis adenylate cyclase have been used in attempts to deliver peptide epitopes to the cell cytosol as internal or amino-terminal fusions. These systems are restricted in their use as potential vaccines because their capacity to deliver larger protein antigens is limited and many individuals have already been immunized against the carrier toxin.
Although peptides are able to stimulate a cellular immune response, whole protein antigens can be better suited for use in an effective vaccine for two reasons. First, the epitope that is essential for protection in one genetic background can prove to be irrelevant in a different genetic background. Therefore, it is beneficial for a broadly applied T cell vaccine to use the full-length protein from which the various relevant epitopes are derived. Second, peptides recognized by cytotoxic T lymphocytes are processed from the whole protein by specialized degradative machinery, including the proteasome complex. In certain instances, the processing of the relevant peptide epitopes is dependent on the flanking amino acid sequences. However, flanking residues are not always important for proper processing. Because it currently is not possible to accurately predict which epitopes are dependent on their context for proper processing, it is important to deliver the entire antigen to the cell cytosol for optimal processing and presentation. Therefore, there is a need for new vaccines/immunogens that are more immunogenic, e. g. immunogens that consist of the whole polypeptide or larger portions thereof and/or novel strategies for introducing the vaccines/immunogen into cells to elicit an immune response.
Adjuvants:
The effectiveness of vaccines is often increased by giving them in adjuvants. An effective immune responses against malignancies and against several infectious pathogens are mediated by T cells. In particular, T helper epitopes are necessary for the induction of high titers of antigen-specific IgG antibodies. An adjuvant or immunostimulator can be used to enhance either humoral or cellular immunity or both. As a result, less recombinant antigen is needed for a standard vaccine or the low-responders respond effectively without increasing the antigen dose.
The primary purpose of an adjuvant is to enhance the immune response to a particular antigen of interest. In the context of antibody production for research purposes, adjuvants stimulate the rapid and sustained production of high titers of antibodies with high avidity. This permits ready recovery of antibody for further research in vitro. Adjuvants have the capability of influencing titer, response duration, isotype, avidity and some properties of cell-mediated immunity. The use of adjuvants is required for many antigens which by themselves are weakly immunogenic.
Adjuvants can increase the immune response to a particular antigen of interest through three basic mechanisms. The first is to enhance long term release of the antigen by functioning as a depot. Long term exposure to the antigen should increase the length of time the immune system is presented with the antigen for processing as well as the duration of the antibody response. The second is the interaction the adjuvant has with immune cells. Adjuvants can act as non-specific mediators of immune cell function by stimulating or modulating immune cells. Adjuvants can also enhance macrophage phagocytosis after binding the antigen as a particulate (a carrier/vehicle function).
The choice of the appropriate adjuvant is exceedingly important from both the aspect of the end result (high antibody response) as well as the immunized animal's welfare. Many of the adjuvants have the capacity to cause inflammation, tissue necrosis and pain in animals, and are thus not suited for human vaccination.
Typically, selection of an adjuvant is based upon antigen characteristics (size, net charge and the presence or absence of polar groups), as well as minimizing discomfort. For many years the only effective adjuvant available was complete Freund's adjuvant (CFA). In the past, adjuvants have also been selected based upon the species to be immunized, as some adjuvants will work better than others depending on the species. However, adjuvant selection remains largely empirical.
Antigens that are easily purified or available in large quantities can be good choices for starting with the least inflammatory adjuvants for immunization. Antigens which are difficult to come by (e.g., very small quantities are available), as well as small molecular weight compounds or weakly immunogenic antigens are more suitable candidates for combination with an adjuvant to increase the immune response.
Typically, adjuvants slow antigen release for a more sustained immune stimulation, bind toll-like receptors on macrophages and dendritic cells to stimulate production of inflammatory cytokines, and/or activate antigen presenting cells to express factors, including IL-10, that stimulate T cell activation. IL-10 is implicated in the adaptive immune response in humans, in particular, in promoting the development of Th2-lymphocytes from naïve (Th0) cells. Th2-lymphocytes recognize antigens presented by B-lymphocytes. Th2 cells, in turn, produce cytokines, including interleukins 2, 4, 5, 10, and 13, which promote antibody production. In addition, the production of IL-4 by Th2 cells enables the animal to make a quick antibody response that is essential to resistance to pathogenic antigens.
Collectively these cytokines enable activated B-lymphocytes to proliferate and stimulate activated B-lymphocytes to synthesize and secrete antibodies, promote the differentiation of B-lymphocytes into antibody-secreting plasma cells, and enable antibody producing cells to switch the class of antibodies being produced. Thus, an adjuvant is an important element of a good immune response to an antigen presented in a vaccine.
Complete Freund's Adjuvant (CFA), a mineral oil emulsion adjuvant, was, for many years, the adjuvant of choice because of the ability of CFA to boost antibody production following vaccination. However, CFA, while immunogenically potent, frequently produced abscesses, granulomas, and tissue sloughs. In addition, multiple exposures to CFA are known cause severe hypersensitivity reactions, and accidental exposure of personnel to CFA can result in sensitization to the associated antigen. Another common adjuvant frequently used for vaccine antigen delivery is aluminum salt (“alum”). Most alum adjuvants are generally weaker adjuvants than emulsion adjuvants, and generally cause only mild inflammatory reactions. However, alum is best used with strongly immunogenic antigens, and is thus not always appropriate.
Furthermore, in order to generate a CMI response, an antigen must be delivered to the interior of the cell. Exogenous proteins are poorly taken up by the cell. Accordingly, the preferred method has been using procedures such as viral vectors, liposomes, naked DNA or a similar approach. However, such approaches have many draw backs. For example, many recombinant viruses generate antigenic reactions themselves, upon repeated administration. Since standard forms of generating immune reactions typically require an initial injection, referred to as the prime, and subsequent injections, referred to as boosts, to achieve a satisfactory immunity, this can be a serious problem. Moreover, while much attention has been placed on improving the safety of viral vectors, there are always certain risks. For example, many of the target populations, such as those infected with HIV, can have a weakened immune system. Thus, certain viral vectors that are perfectly safe in many individuals can pose some degree of risk to these individuals.
It would be desirable to use an adjuvant which assist in facilitating delivery of an antigen into the interior of a cell in order to generate a CMI response.